Orthoester-based surfactants and associated methods

ABSTRACT

Provided are treatment fluids that comprise a base fluid and an orthoester-based surfactant. In some instances, the treatment fluid may have a pH of about 8.5 or greater. Also provided are emulsified treatment fluids that comprise an oleaginous phase, an aqueous phase having a pH of about 8.5 or greater, and an orthoester-based surfactant. Methods of using the treatment fluid and methods of facilitating flow through a conduit also are provided.

The present invention is related to co-pending U.S. application Ser. No.______ [Attorney Docket No. HES 2004-IP-015213U2] entitled“Orthoester-Based Surfactants and Associated Methods,” filedconcurrently herewith, the entire disclosure of which is incorporatedherein by reference.

BACKGROUND

The present invention relates to orthoester-based surfactants. Moreparticularly, the present invention relates to treatment fluidscomprising orthoester-based surfactants and associated methods.

Surfactants may be used in a variety of applications. Such applicationsmay be above or below ground, for example, in a subterranean formationpenetrated by a well bore. Where used in above-ground applications,surfactants may be used, for example, to emulsify an oleaginous fluid(e.g., a heavy oil) to facilitate transport in a pipeline. Surfactantsalso may be used in subterranean applications, for example, in drillingoperations, stimulation treatments (e.g., fracturing treatments), wellbore cleanups, viscous sweeps, and completion operations (e.g., sandcontrol treatments, gravel packing). In these applications, thesurfactants may be used for a number of purposes, including asemulsifying agents, non-emulsifying agents, foaming agents, defoamingagents, dispersants, wetting agents, and the like.

While a variety of surfactants have been used in subterraneanapplications, problems have been associated with their use. Forinstance, certain surfactants used heretofore may have undesirableenvironmental characteristics and/or may be limited by strictenvironmental regulations in certain areas of the world. Thus,degradable surfactants have been used to reduce the potential forbioaccumulation and/or persistence of such surfactants in theenvironment. Currently available degradable surfactants, such as esters,amides, and acetals, have characteristics that may limit theirusefulness in subterranean applications. For instance, esters and amidesmay not degrade as desired in conditions that may be encountereddownhole. Further, degradation of esters and amides is slowest at pHsencountered in the subterranean environment, and thus may result inunacceptably long persistence times. Likewise, problems also may beencountered with the use of acetals in subterranean applications. Whileacetals are usually stable at high pH values (e.g., >about 6), low pHvalues (e.g., about 1-4) may be required for their degradation to occurat desirable rates. This typically requires exposure of the surfactantto an acid to facilitate the degradation thereof after introduction intothe subterranean formation, which may add undesired expense andcomplexity to the subterranean application.

SUMMARY

The present invention relates to orthoester-based surfactants. Moreparticularly, the present invention relates to treatment fluidscomprising orthoester-based surfactants and associated methods. As usedherein, the term “orthoester-based surfactant” refers to amphiphiliccompounds that contain one or more orthoester functional groups.

In one embodiment, the present invention provides a method thatcomprises: providing a treatment fluid comprising a base fluid and anorthoester-based surfactant; and introducing the treatment fluid into asubterranean formation.

Another embodiment of the present invention provides a methodcomprising: providing an emulsified treatment fluid comprising anoleaginous phase, an aqueous phase having a pH of about 8.5 or greater,and an orthoester-based surfactant; and introducing the emulsifiedtreatment fluid into a subterranean formation.

Another embodiment of the present invention provides a method offacilitating flow through a conduit comprising: providing an emulsifiedtreatment fluid comprising an orthoester-based surfactant, an oleaginousphase, and an aqueous phase having a pH of about 8.5 or greater; flowingthe emulsified treatment fluid through the conduit; and reducing the pHof the aqueous phase so as to facilitate degradation of at least aportion of the orthoester-based surfactant, thereby facilitatingseparation of the oleaginous phase and the aqueous phase.

Yet another embodiment of the present invention provides a method ofsynthesizing an orthoester-based surfactant comprising: providing adiketene acetal or a multiketene acetal, and reacting the diketeneacetal or the multiketene acetal with one or more hydrophobic alcoholsand one or more hydrophilic alcohols.

The features and advantages of the present invention will be readilyapparent to those skilled in the art upon a reading of the descriptionof the preferred embodiments that follows.

DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention relates to orthoester-based surfactants. Moreparticularly, the present invention relates to treatment fluidscomprising orthoester-based surfactants and associated methods. Thesetreatment fluids may be suitable for use in a variety of subterraneanapplications, including, but not limited to, drilling operations,stimulation treatments (e.g., fracturing treatments), well borecleanups, viscous sweeps, and completion operations (e.g., sand controltreatments, gravel packing). These treatment fluids also may be usedabove ground, for example, to facilitate the transport of an oleaginousfluid. As used herein, the term “treatment,” or “treating,” refers toany operation that uses a fluid in conjunction with a desired functionand/or for a desired purpose. The term “treatment,” or “treating,” doesnot imply any particular action by the fluid.

I. Treatment Fluids of the Present Invention

The treatment fluids of the present invention generally comprise a basefluid and an orthoester-based surfactant. These treatment fluids mayinclude aqueous fluids, aqueous gels, viscoelastic surfactant gels, oilgels, foamed gels, and emulsions (e.g., water-in-oil or oil-in-water).The orthoester-based surfactants included in these treatment fluids aregenerally degradable, wherein the degradation of the orthoester linkagestherein generally should result in the destruction, or reduction, of thesurfactant's surface activity. The rates of the orthoester-basedsurfactant's degradation should increase with decreasing pH andincreasing temperature. However, these orthoester-based surfactantsgenerally are stable at a pH of about 8.5 or greater. Among otherthings, the orthoester-based surfactants may be included in thetreatment fluids of the present invention for any of a number ofpurposes, including as emulsifying agents, non-emulsifying agents,foaming agents, defoaming agents, dispersants, wetting agents,combinations thereof, and the like.

The base fluids included in the treatment fluids of the presentinvention may include any fluid that may be used to prepare a suitabletreatment fluid. Examples of suitable base fluids include, but are notlimited to aqueous fluids, oleaginous fluids, and combinations thereof.Examples of suitable oleaginous fluids include, but are not limited to,olefins, internal olefins, alkanes, aromatic solvents, cycloalkanes,liquefied petroleum gas, kerosene, diesel oils, crude oils, heavy oils,gas oils, fuel oils, paraffin oils, mineral oils, low toxicity mineraloils, olefins, esters, amides, synthetic oils (e.g., polyolefins),polydiorganosiloxanes, siloxanes, organosiloxanes, ethers, acetals,dialkylcarbonates, hydrocarbons, diesel, crude oil, organic solvents,oil, and combinations thereof. Because the orthoester-based surfactantsgenerally degrade with reduced pH, a treatment with an aqueous-basefluid should, in most embodiments, have a pH of about 8.5 or greater.Suitable aqueous liquids may include, but are not limited to,freshwater, seawater, saltwater, brines (e.g., natural or formulatedbrines), and combinations thereof. The aqueous liquid may be from anysource, provided that it does not contain an excess of compounds thatmay adversely affect the emulsified treatment fluid. In someembodiments, nonaqueous fluids also may be used as the base fluid. Incertain embodiments, the aqueous liquid may comprise a gelling agent(e.g., viscoelastic surfactants, natural or synthetic polymers, etc.)for gelling the aqueous liquid and increasing its viscosity, and,optionally, a crosslinking agent for crosslinking the gelling agent andfurther increasing the viscosity of the fluid.

The orthoester-based surfactants useful in the present inventiongenerally may be any suitable orthoester-based surfactant. In certainembodiments, at least a portion of the orthoester-based surfactants areof Formula I:

wherein R is hydrogen or an alkyl group having from 1 to 4 carbons, R¹is a hydrophobic group, R² is a hydrophilic group, R³ is CH₃, C₂H₅,C₃H₇, or C₄H₉, x is an integer from 1 to 2, y is an integer from 0 to 2,and z is an integer from 0 to 2, wherein the summation of x, y, and z isequal to 3. Preferably, R³ is CH₃ or C₂H₅.

In certain embodiments, suitable orthoester-based surfactants may besynthesized from diketene acetals or multiketene acetals. In certainembodiments, suitable nonionic or cationic orthoester-based surfactantscontaining an amine may be synthesized by transesterification of analkanolamine and a hydrophobic alcohol with a low molecular weightorthoester, or by the reaction of an alkanolamine and a hydrophobicalcohol with a diketene acetal or a multiketene acetal. Suitableorthoester-based surfactants and methods of synthesis are described inU.S. Pat. No. 6,653,395, the relevant disclosure of which isincorporated herein by reference. Other suitable orthoester-basedsurfactants and methods of synthesis are described in WO 03/018534 andUnited States PG Pub. No. 2004/0039235, the relevant disclosures ofwhich are incorporated herein by reference.

The hydrophobic groups contained in the orthoester-based surfactants maybe any hydrophobic groups derived from hydrophobic alcohols that arecapable of undergoing a transesterification reaction with orthoesters.In certain embodiments, the hydrophobic groups may comprise monoalcoholsderived from fatty alcohols, fatty alcohol ethoxylates, end-cappedhydrophobic poly(alkylene oxides), poly(tetrahydrofuran), andpolybutadiene hydroxyl terminated. Examples of suitable hydrophobicpoly(alkylene oxides) include, for example, end-capped poly(butyleneoxide), and end-capped poly(propylene oxide). Suitable hydrophobicgroups include, for example, alkyl groups with 8 or more carbons.Suitable examples of alkyl groups with 8 or more carbons include, butare not limited to, 2-ethylhexyl, octyl, decyl, coco alkyl, lauryl,oleyl, rape seed alkyl, and tallow alkyl. Those of ordinary skill in theart, with the benefit of this disclosure, will be able to determinesuitable hydrophobic groups based on, among other things, thehydrophilic-lipophilic balance and the particular function that thesurfactant will perform.

The hydrophilic groups contained in the orthoester-based surfactants maybe derived from hydrophilic alcohols. Suitable hydrophilic alcoholsinclude, for example, end-capped poly(ethylene oxide) and alkanolamines.Examples of suitable alkanolamines include, but are not limited to,N,N-dimethylethanolamine; N,N-diethylethanolamine;N-methyldiethanolamine; N-methyl-hydroxyethylpiperazine;1,4-bis(2-hydroxyethyl)piperazine;N,N-dimethyl-N′,N′-bis(2-hydroxyethyl)propylenediamine;N,N-diethyl-N′,N′-bis(2-hydroxyethyl)propylenediamine;3-dimethylamino-1-propanol; 3-diethylamino-1-propanol; andtriethanolamine. In certain embodiments, the hydrophilic groups may bederived from tertiary amines. Hydrophilic groups derived from tertiaryamines may be suitable, in certain embodiments, because the tertiaryamine may be less likely to cause undesired side reactions in thesynthesis of the orthoester-based surfactant. In some embodiments, theorthoester-based surfactant containing tertiary amines may bequaternized by alkylation to form a quaternary ammonium compound acationic orthoester-based surfactant.

In certain embodiments, suitable orthoester-based surfactants of FormulaI may be synthesized by reacting, in one or more steps, a low molecularweight orthoester of Formula II, as illustrated below, with hydrophilicalcohols and hydrophobic alcohols in the presence of an acid catalyst:

wherein R has the same meaning as from Formula I and R⁴ is an alkylgroup having from 1 to 6 carbons. Examples of suitable acid catalystsinclude methanesulphonic acid, p-toluenesulphonic acid, and citric acid.Examples of suitable low molecular weights orthoesters of Formula IIinclude trimethyl orthoformate, trimethyl orthoacetate, triethylorthoformate, triethyl orthoacetate, tripropyl orthoformate; andtripropyl orthoacetate. Low molecular weight orthoesters may be used dueto the ease of transesterification undergone by these molecules withhigh molecular weight alcohols. Examples of suitable hydrophobicalcohols and hydrophilic alcohols are described above. In oneembodiment, a suitable orthoester-based surfactant of Formula I may besynthesized by reacting trimethyl orthoformate with a mixture ofhydrophilic and hydrophobic alcohols in the presence of an acid catalystas illustrated in Reaction Scheme I:

wherein R has the same meaning as from Formula I, R⁵ is a hydrophobicgroup, R⁶ is a hydrophilic group, r is an integer from 1 to 2, p is aninteger from 0 to 2, and q is an integer from 0 to 2, wherein thesummation of r, p, and q is equal to 3. Because the trimethylorthoformate molecule has three positions that may be substituted by thereactants, the product of the reaction depicted in Reaction Scheme I isa mixture of several different reaction products. Some molecules willhave one hydrophobic group substituent and one hydrophilic groupsubstituent. Some molecules will have two hydrophobic groupsubstituents. And some molecules will have two hydrophilic groupsubstituents. The surface activity of the reaction products will depend,among other things, on the substituents, for example, moleculescontaining one hydrophobic group and one hydrophilic group in the samemolecule should show surface activity. Generally, reaction productscontaining only the hydrophilic or the hydrophobic groups will shownegligible to no surface activity.

In certain embodiments, suitable orthoester-based surfactants may besynthesized by reacting, in one or more steps, a diketene, ormultiketene, acetal with one or more hydrophobic alcohols and one ormore hydrophilic alcohols. Examples of suitable hydrophobic alcohols andhydrophilic alcohols that may be used in the synthesis of anorthoester-based surfactant from a diketene, or multiketene, acetal aredescribed previously. Examples of suitable diketene, or multiketene,acetals may be synthesized as described in U.S. Pat. No. 4,304,767, therelevant disclosure of which is incorporated herein by reference. In oneembodiment, as illustrated below in Reaction Scheme II, a diketeneacetal may be synthesized by reacting pentaerythritol andchloroacetaldehyde dimethyl acetal in the presence of p-toluenesulfonicacid or methanesulfonic acid to afford 2 which on dehydrohalogenation inpresence of t-butoxide in t-butanol afford diketene acetal 3, and asuitable orthoester-based surfactant 4 may be synthesized by reactingthe resultant diketene acetal with one mole equivalent each of thehydrophobic and hydrophilic alcohols in the presence of a small amountof iodine dissolved in pyridine. In some embodiments, theorthoester-based surfactant may be synthesized by mixing the hydrophobicand hydrophilic alcohols with the diketene acetal without the aid of aniodine/pyridine catalyst, provided the alcohols and diketene acetals areextremely pure.

wherein R⁷ is a hydrophobic group and R⁸ is a hydrophilic group. Asthose of ordinary skill in the art will appreciate, the product of thereaction depicted in Reaction Scheme II is a mixture of severaldifferent reaction products. Some molecules will have one hydrophobicgroup substituent and one hydrophilic group substituent. Some moleculeswill have two hydrophobic group substituents. And some molecules willhave two hydrophilic group substituents. The surface activity of thereaction products will depend, among other things, on the substituents,for example, molecules containing one hydrophobic group and onehydrophilic group in the same molecule should show surface activity.Generally, reaction products containing only the hydrophilic or thehydrophobic groups will show negligible to no surface activity.

In certain embodiments, a suitable orthoester-based surfactantcontaining an amine may be synthesized by transesterification reactionof an alkanolamine and a hydrophobic alcohol with a low molecular weightorthoester of the previously listed Formula II, or by reaction of analkanolamine and a hydrophobic alcohol with a diketene acetetal or amultiketene acetal. In certain embodiments, the orthoester-basedsurfactant containing an amine may be synthesized from a tertiary amine.In some embodiments, the orthoester-based surfactant containing atertiary amine may be quaternized by alkylation to form a quaternaryammonium compound, for example, a cationic orthoester-based surfactant.

In some embodiments, the amine may be a tertiary amine or a quaternaryammonium salt. For example, a tertiary amine may be quarternized byreacting the orthoester-based surfactant containing a tertiary aminewith an alkylating agent (e.g., methyl chloride or dimethylsulfate) toobtain a cationic orthoester-based surfactant. Suitable alkanolamines,low molecular weight orthoesters, and diketene acetals are describedpreviously.

The particular function that the orthoester-based surfactants useful inthe present invention will perform depends on a variety of factors.These factors may include, but are not limited to, the choice ofhydrophobic and hydrophilic groups, the relative amounts of hydrophobicand hydrophilic groups and temperature. For example, whether anoil-in-water (“o/w”) or a water-in-oil (“w/o”) emulsion is formed may bedetermined by the relative hydrophobicity of the surfactant tail and thehydrophilicity of the head group. The hydrophilic/lipophilic balance(“HLB”) of the surfactant may provide a quantitative prediction ofwhether the surfactant will facilitate the formation of an o/w or a w/oemulsion. The HLB may be determined from the chemical formula of thesurfactant using empirically determined group numbers. Even the HLBmethod is only semi-empirical and other factors (such as the relativephase volumes of oil and water) may have a considerable influence on thetype of emulsion formed.

By varying the above-listed factors, the specific properties of theorthoester-based surfactant such as solubility, wettability,emulsifying, foaming, defoaming, cloud point, gelling, detergency, andthe like may be varied. For example, where used as an emulsifying agent,an orthoester-based surfactant having an HLB of from about 3 to about 6may be suitable to stabilize a w/o emulsion. In other embodiments, forstabilizing an o/w emulsion, an orthoester-based surfactant having anHLB of from about 8 to about 18 may be suitable. Those of ordinary skillin the art, with the benefit of this disclosure, will be able todetermine the appropriate orthoester-based surfactant to use for aparticular application.

The amount of the orthoester-based surfactants useful in the presentinvention to include in the treatment fluids of the present inventionwill vary dependent upon, among other things, the particularapplication. In certain embodiments, the orthoester-based surfactantsshould be present in an amount in the range of from about 0.01% to about5% by weight of the fluid. In certain embodiments, the orthoester-basedsurfactants should be present in an amount in the range of from about0.05% to about 2% by weight of the fluid.

In some embodiments, where an aqueous-base fluid is used, the pH of thetreatment fluids of the present invention may, among other things,affect the degradation of the orthoester-based surfactants. Theorthoester-based surfactants useful in the present invention aregenerally stable at a pH of about 8.5 or greater. However, degradationof the orthoester linkage should generally result in destruction of thesurface activity of the surfactants. At a pH of about 8 or less theorthoester-based surfactants should degrade at reasonable rates.Generally, a reduction in pH to neutral value (about pH 7) should allowthe orthoester-based surfactant to degrade relatively quickly. The ratesof degradation should increase with decreasing pH and increasingtemperature. In subterranean applications, the buffering action of theformation together with temperature may, in some embodiments, providethe desired degradation of the orthoester-based surfactant. Generally,the pH of a particular treatment fluid of the present invention may bemaintained at about 8.5 or greater. In the embodiments where anonaqueous base fluid is used, the orthoester-based surfactant presentin the treatment fluid should begin to degrade upon interaction withformation fluids (e.g., formation brines).

To maintain the pH of the treatment fluids of the present invention in adesired range if necessary, the fluids optionally may comprise a pHbuffer and/or a strong base. The pH buffer and/or strong base may beincluded in the treatment fluids to adjust pH to, and/or maintain pH in,a desired range, among other things, for the stability of theorthoester-based degradable surfactants. In some embodiments, a pHbuffer may be used, for example, where an acid will be used to lower thepH of the treatment fluid at a desired time. Examples of suitable pHbuffers include, but are not limited to, sodium carbonate, potassiumcarbonate, sodium or potassium diacetate, sodium or potassium phosphate,sodium or potassium hydrogen phosphate, and combinations thereof. Insome embodiments, a strong base may be used, for example, where theformation buffering effect will be used to lower the pH of the treatmentfluid. Examples of suitable strong bases include, but are not limitedto, sodium hydroxide, potassium hydroxide, and combinations thereof. ThepH buffer and/or strong base may be present in the treatment fluids ofthe present invention in an amount sufficient to maintain the pH of thetreatment fluid at or above about 8.5 as desired. One of ordinary skillin the art, with the benefit of this disclosure, will recognize theappropriate pH buffer and/or strong base and amount thereof to use for achosen application.

While the orthoester-based surfactants included in the treatment fluidsof the present invention are generally degradable, or cleavable, it maybe desired, in some embodiments, for a faster degradation. Therefore, insome embodiments, to facilitate the degradation of the orthoester-basedsurfactants and thus generally destroy or reduce their surface activity,the pH of the treatment fluid may be decreased at a desired time.Generally, at a pH of about 8 or less the orthoester-based surfactantsshould degrade at reasonable rates. In subterranean applications, thebuffering action of the formation together with temperature may, in someembodiments, provide the desired degradation. In some embodiments, theorthoester-based surfactants may be included in the treatment fluid tofacilitate the formation of an emulsion. At a desired time, the emulsionmay be broken by decreasing the pH of the treatment fluid's aqueousphase sufficiently to allow for the degradation of the orthoester-basedsurfactant. Upon degradation of the orthoester linkages, the reductionin surface activity of the orthoester-based surfactant will facilitate abreak of the emulsion. Separation of the two phases of an emulsion iscommonly referred to as “breaking” the emulsion.

To reduce the pH of the treatment fluid at a desired time a number ofmethods may be employed. In some embodiments, the treatment fluid may becontacted by an acid after introduction of the treatment fluid into thesubterranean formation. Examples of suitable acids include, but are notlimited to, hydrochloric acid, hydrofluoric acid, formic acid,phosphoric acid, sulfamic acid, acetic acid, derivatives thereof, andmixtures thereof.

In other embodiments a delayed-release acid, such as an acid-releasingdegradable material or an encapsulated acid, may be included in thetreatment fluid so as to reduce the pH of the treatment fluid at adesired time, for example, after introduction of the treatment fluidinto the subterranean formation. Suitable encapsulated acids that may beincluded in the treatment fluids of the present invention include butare not limited to, fumaric acid, formic acid, acetic acid, aceticanhydride, anhydrides, hydrochloric acid, hydrofluoric acid,hydroxyfluoboric acid, combinations thereof, and the like. Exemplaryencapsulation methodology is described in U.S. Pat. Nos. 5,373,901;6,444,316; 6,527,051; and 6,554,071, the relevant disclosures of whichare incorporated herein by reference. Acid-releasing degradablematerials also may be included in the treatment fluids of the presentinvention to decrease the pH of the fluid. Suitable acid-releasingdegradable materials that may be used in conjunction with the presentinvention are those materials that are substantially water insolublesuch that they degrade over time, rather than instantaneously, in anaqueous environment to produce an acid. Examples of suitableacid-releasing degradable materials include esters, polyesters,orthoesters, polyorthoesters, lactides, polylactides, glycolides,polyglycolides, substituted lactides wherein the substituted groupcomprises hydrogen, alkyl, aryl, alkylaryl, acetyl heteroatoms andmixtures thereof, substantially water insoluble anhydrides,poly(anhydrides), and mixtures and copolymers thereof. Materialssuitable for use as an acid-releasing degradable material of the presentinvention may be considered degradable if the degradation is due, interalia, to chemical processes, such as hydrolysis, oxidation, or enzymaticdecomposition. The appropriate pH-adjusting agent or acid-releasingmaterial and amount thereof may depend upon the formationcharacteristics and conditions, the particular orthoester-basedsurfactant chosen, and other factors known to individuals skilled in theart, with the benefit of this disclosure.

Depending on the particular application, the treatment fluids of thepresent invention may further comprise any of a variety of additionaladditives. Examples of suitable additives include, but are not limitedto, proppant particulates, gravel particulates, weighting agents,organophilic clays, bridging agents, fluid loss control agents, wettingagents, corrosion inhibitors, scale inhibitors, fluid loss controladditives, gas, paraffin inhibitors, asphaltene inhibitors, catalysts,hydrate inhibitors, iron control agents, clay control agents, biocides,friction reducers, combinations thereof and the like. The particularadditives included in the treatment fluids should not adversely affectother components of the treatment fluid or the orthoester-basedsurfactant. Individuals skilled in the art, with the benefit of thisdisclosure, will recognize the types of additives to include for aparticular application.

The treatment fluids of the present invention may be used in anysuitable subterranean application. Such applications may include, butare not limited to, drilling operations, stimulation treatments (e.g.,fracturing treatments), and completion operations (e.g., sand controltreatments, gravel packing). An example method of the present inventioncomprises: providing a treatment fluid of the present invention thatcomprises a base fluid and an orthoester-based surfactant, introducingthe treatment fluid into a subterranean formation. Introducing thetreatment fluid into the subterranean fluid may include introduction ofthe treatment fluid into a well bore that penetrates the subterraneanformation. In some embodiments, the method may further comprise allowingat least a portion of the orthoester-based surfactant to degrade. Aspreviously discussed, at a desired time, the pH of the treatment fluidmay be reduced so as to facilitate the degradation of theorthoester-based surfactant, thereby destroying or reducing the surfaceactivity of the surfactant.

In the drilling embodiments, the treatment fluids may be used indrilling at least a portion of a well bore that penetrates thesubterranean formation. For example, the treatment fluids may be used asa drilling fluid or a drill-in fluid. In another embodiment, thetreatment fluids of the present invention may be used in a sand controltreatment (e.g., as a gravel-packing fluid). In the sand controlembodiments, the treatment fluids may further comprise gravelparticulates, wherein at least a portion of the gravel particulates maybe deposited within or adjacent to a portion of the subterraneanformation to form a gravel pack. As referred to herein, the term “gravelpack” refers to gravel particulates that have been deposited in a wellbore so as to provide at least some degree of sand control, such as bypacking the annulus between the well bore and a screen disposed in thewell bore with the gravel particulates of a specific size designed toprevent the passage of formation sand. In some embodiments, screenlessgravel packs also may be employed. In the fracturing embodiments, thetreatment fluid may be introduced into the subterranean formation at orabove pressure sufficient to create or enhance one or more fractures inthe subterranean formation.

II. Emulsified Embodiments of the Treatment Fluids of the PresentInvention

For the emulsion embodiments, the treatment fluids of the presentinvention may be an emulsified treatment fluid that comprises anoleaginous phase, an aqueous phase having a pH of about 8.5 or greater,and an orthoester-based surfactant. As referred to herein, the term“emulsified treatment fluid” refers to any emulsified fluid that has acontinuous phase and a discontinuous phase, whether a w/o emulsion or ano/w emulsion. In an o/w emulsion, the aqueous phase is the continuous(or external) phase and the oleaginous phase is the discontinuous (orinternal) phase. In a w/o emulsion (or invert emulsion), the aqueousphase is the discontinuous phase and the oleaginous phase is thecontinuous phase.

Generally, the emulsified treatment fluids of the present invention aresuitable for use in a variety of applications where an o/w emulsion or aw/o emulsion is suitable. For example, the orthoester-based surfactantsmay be useful for facilitating the formation of w/o emulsions that maybe used in a variety of subterranean operations including, drillingoperations (e.g., as a drilling fluid or a drill-in fluid), fracturingtreatment (e.g., as a fracturing fluid), well bore cleanups, viscoussweeps, and sand control treatments (e.g., as a gravel-packing fluid). Adrill-in fluid is a drilling fluid formulated for drilling the reservoirportion of the subterranean formation. Also, the orthoester-basedsurfactants may be useful for facilitating the formation of an o/wemulsion that may be used in a variety of subterranean applications,including well bore cleanups, such as for the removal of paraffin,asphaltene, and/or scale deposits. In another embodiment, theorthoester-based surfactant may be used to form an emulsified treatmentfluid that may be used to facilitate the transport of the oleaginousphase, for example, to facilitate the flow of heavy oil through apipeline. As used herein, the term “heavy oil” refers to any petroleumwith an API gravity of less than 28 degrees or a high specific density.

The oleaginous phase of the emulsified treatment fluids of the presentinvention may comprise any oleaginous fluid suitable for use inemulsions used in subterranean applications. The oleaginous fluid may befrom natural or synthetic sources. Examples of suitable oleaginousfluids include, but are not limited to, α-olefins, internal olefins,alkanes, aromatic solvents, cycloalkanes, liquefied petroleum gas,kerosene, diesel oils, crude oils, heavy oils, gas oils, fuel oils,paraffin oils, mineral oils, low toxicity mineral oils, olefins, esters,amides, synthetic oils (e.g., polyolefins), polydiorganosiloxanes,siloxanes, organosiloxanes, ethers, acetals, dialkylcarbonates,hydrocarbons, and combinations thereof.

The amount of the oleaginous phase to include in the emulsifiedtreatment fluid depends on a number of factors, including the particularorthoester-based surfactant used, the type of emulsion (e.g., o/w orw/o), the desired application, and rheology. For example, in certainembodiments, such as stimulation, the emulsified treatment fluid shouldhave sufficient viscosity for proppant transport. In some embodiments,for an o/w emulsion, the oleaginous phase may be present in an amount inthe range of from about 10% to about 65% by volume of the emulsifiedtreatment fluid. In some embodiments, for a w/o emulsion, the oleaginousphase may be present in an amount in the range of from about 20% toabout 90% by volume of the emulsified treatment fluid.

For the emulsion embodiments, the emulsified treatment fluids of thepresent invention may also comprise an aqueous phase. Generally, theaqueous phase may comprise an aqueous liquid. Suitable aqueous liquidsmay include, but are not limited to, freshwater, seawater, saltwater,brines (e.g., natural or formulated brines), and combinations thereof.The aqueous liquid may be from any source, provided that it does notcontain an excess of compounds that may adversely affect the emulsifiedtreatment fluid.

The amount of the aqueous phase to include in the emulsified treatmentfluid depends on a number of factors, including the particularorthoester-based surfactant used, the type of emulsion (e.g., o/w orw/o), the desired application, and rheology. In some embodiments, for ano/w emulsion, the aqueous phase may be present in an amount in the rangeof from about 35% to about 90% by volume of the emulsified treatmentfluid. In some embodiments, for a w/o emulsion, the aqueous phasegenerally may be present in an amount in the range of from about 10% toabout by 80% volume of the emulsified treatment fluid.

Generally, the orthoester-based surfactant should be included in theemulsified treatment fluids of the present invention so as to reduce thesurface tension between the oleaginous phase and the aqueous phase so asto facilitate the formation and stabilization of the emulsifiedtreatment fluids of the present invention. As discussed previously, theorthoester-based surfactant may be tailored so as to facilitate theformation of an o/w or a w/o emulsion. The orthoester-based surfactantmay be present in the emulsified treatment fluids of the presentinvention in amount in the range of from about 0.01% to about 5% byweight of the emulsified treatment fluids. In another embodiment, theorthoester-based surfactant may be present in the emulsified treatmentfluids of the present invention in an amount in the range of from about1% to about 3% by weight of the emulsified treatment fluids.

As previously discussed, the orthoester-based surfactants included inthe emulsified treatment fluids of the present invention are generallystable at a pH of about 8.5 or greater. As a result, for stabilizationof the orthoester-based surfactants, the aqueous phase of the emulsifiedtreatment fluids of the present invention should have a pH of about 8.5or greater. To maintain the pH of the treatment fluids of the presentinvention in a desired range, the aqueous phase optionally may comprisea pH buffer and/or a strong base. The pH buffer and/or strong base maybe included in the aqueous phase to adjust the pH, and/or maintain thepH in, a desired range, among other things, for the stability of theorthoester-based degradable surfactants. In some embodiments, a pHbuffer may be used, for example, where an acid will be used to lower thepH of the treatment fluid at a desired time. In some embodiments, astrong base may be used, for example, where the formation bufferingeffect will be used to lower the pH of the treatment fluid. Suitable pHbuffers and strong bases may include those described previously. The pHbuffer and/or strong base may be present in the aqueous phase of thepresent invention in an amount sufficient to maintain the pH of thetreatment fluid at or above about 8.5 as desired. One of ordinary skillin the art, with the benefit of this disclosure, will recognize theappropriate pH buffer and/or strong base and amount thereof buffer touse for a chosen application.

Because the orthoester-based surfactants act to stabilize the emulsifiedtreatment fluids of the present invention, the emulsified treatmentfluids should have a delayed break, due to the degradation of thesurfactants therein, without the need for inclusion of conventionalemulsion breakers (e.g., acid precursors) therein. In subterraneanapplications, the buffering action of the formation together withtemperature may, in some embodiments, provide the desired degradation.However, in some embodiments, a faster break may be desired so that adelayed-release acid, such as an acid-releasing degradable material oran encapsulated acid, may be included in the emulsified treatmentfluids. Suitable delayed-release acids may include those describedpreviously. In some embodiments, the emulsified treatment fluids of thepresent invention may be contacted by an acid after introduction of theemulsified treatment fluid into the well bore. Examples of suitableacids include, but are not limited to, hydrochloric acid, hydrofluoricacid, formic acid, phosphoric acid, sulfamic acid, acetic acid,derivatives thereof, and mixtures thereof.

Depending on the particular application, the emulsified treatment fluidsof the present invention may further comprise any of a variety ofadditional additives. Examples of suitable additives include, but arenot limited to, proppant particulates, gravel particulates, weightingagents, organophilic clays, bridging agents, fluid loss control agents,wetting agents, corrosion inhibitors, scale inhibitors, fluid losscontrol additives, gas, paraffin inhibitors, asphaltene inhibitors,catalysts, hydrate inhibitors, iron control agents, clay control agents,biocides, friction reducers, combinations thereof, and the like. Theparticular additives included in the treatment fluids should notadversely affect other components of the emulsified treatment fluid.Individuals skilled in the art, with the benefit of this disclosure,will recognize the types of additives to include for a particularapplication.

The emulsified treatment fluids of the present invention may be used inany suitable subterranean application where an emulsion may be used,including, but not limited to, drilling operations (e.g., as a drillingfluid or a drill-in fluid), fracturing treatment (e.g., as a fracturingfluid), well bore cleanups, viscous sweeps, and sand control treatments(e.g., as a gravel-packing fluid). An example method of the presentinvention comprises: providing an emulsified treatment fluids of thepresent invention that comprises an orthoester-based surfactant, anoleaginous phase, and an aqueous phase having a pH of about 8.5 orgreater; and introducing the emulsified treatment fluid into asubterranean formation. Introducing the emulsified treatment fluid intothe subterranean formation includes introducing the emulsified treatmentfluid into a well bore that penetrates the subterranean formation. Aspreviously discussed, the emulsified treatment fluid may be an o/w or aw/o emulsion.

In the drilling embodiments, the emulsified treatment fluids may be usedin drilling at least a portion of a well bore that penetrates thesubterranean formation. For example, the emulsified treatment fluids maybe used as a drilling fluid or a drill-in fluid. In another embodiment,the emulsified treatment fluids of the present invention may be used ina sand control treatment (e.g., as a gravel-packing fluid). In the sandcontrol embodiments, the emulsified treatment fluids may furthercomprise gravel particulates, wherein at least a portion of the gravelparticulates may be deposited within or adjacent to a portion of thesubterranean formation to form a gravel pack. In the fracturingembodiments, the emulsified treatment fluid may be introduced into thesubterranean formation at or above pressure sufficient to create orenhance one or more fractures in the subterranean formation.

Another example of a method of the present invention comprises utilizingthe emulsified treatment fluids of the present invention to facilitatethe flow of the oleaginous phase through a conduit, for example, tofacilitate the flow of heavy oil through a pipeline. An example of sucha method may comprise: providing an emulsified treatment fluid of thepresent invention that comprises an orthoester-based surfactant, anoleaginous phase, and an aqueous phase having a pH of about 8.5 orgreater; flowing the emulsified treatment fluid through a conduit;reducing the pH of the aqueous phase so as to facilitate degradation ofat least a portion of the orthoester-based surfactant, therebyfacilitating the separation of the oleaginous phase and the aqueousphase. Since the emulsified treatment fluid may be broken by degradationof the orthoester-based surfactant, when the pH of the aqueous phase isreduced, the oleaginous phase and the aqueous phase should separate.Among other things, this may allow for recovery of the oleaginous fluidat a desired location. In most embodiments, the pH of the aqueous phasemay reduced at the receiving end of the pipeline where desired torecover the oleaginous fluid. In certain embodiments, the oleaginousphase may be a heavy oil. Where a heavy oil is used, the emulsifiedtreatment fluid may be used to facilitate the flow of the heavy througha conduit, such as a pipeline.

To facilitate a better understanding of the present invention, thefollowing examples of preferred embodiments are given. In no way shouldthe following examples be read to limit, or define, the scope of theinvention.

EXAMPLE 1

A 75/25 o/w emulsion was prepared in accordance with the followingprocedure. First, 2 grams (g) of an orthoester-based surfactant inaccordance with Formula I (R¹=octadecyl, R²=MPEG-350, and R³=CH₃) weredissolved in 50 milliliters (mL) of a solution comprising 0.05% sodiumhydroxide by weight of the solution. To the resulting solution, 150 mLof mineral oil (HDF-2000) were added while shearing in a blender to forman o/w emulsion. The rheology of the o/w emulsion prepared in accordancewith the above procedure was measured on a FANNY 35 viscometer fittedwith a yield stress adapter. Yield stress adapters are described in U.S.Pat. No. 6,874,353, the relevant disclosure of which is incorporatedherein by reference. Data analysis was done with the Casson model todetermine that: yield stress=13 Pascals; infinite shear rateviscosity=97 centipoise; and r²=0.99, wherein r is the correlationcoefficient.

EXAMPLE 2

A 75/25 w/o emulsion was prepared in accordance with the followingprocedure. First, 4 g of an orthoester-based surfactant in accordancewith Formula I (R¹=butyl capped PPO-2500, R²=MPEG-350, and R³=CH₃) weredissolved in 50 mL of a mineral oil (HDF-2000) containing 10% ofACCOLADE™ drilling fluid base oil by volume. ACCOLADE™ drilling fluidbase oil is commercially available from Halliburton Energy Services,Inc., Duncan Okalahoma. To the resulting solution, 150 mL of water wereadded while shearing in a blender to form a w/o emulsion. The rheologyof the w/o emulsion prepared in accordance with the above procedure wasmeasured on a FANN® 35 viscometer fitted with a yield stress adapter.Yield stress adapters are described in U.S. Pat. No. 6,874,353, therelevant disclosure of which is incorporated herein by reference. Dataanalysis was done with the Casson model to determine that: yieldstress=40 Pascals; infinite shear rate viscosity=153 centipoise; andr²=0.99, wherein r is the correlation coefficient.

Therefore, the present invention is well adapted to attain the ends andadvantages mentioned as well as those that are inherent therein. Theparticular embodiments disclosed above are illustrative only, as thepresent invention may be modified and practiced in different butequivalent manners apparent to those skilled in the art having thebenefit of the teachings herein. Furthermore, no limitations areintended to the details of construction or design herein shown, otherthan as described in the claims below. It is therefore evident that theparticular illustrative embodiments disclosed above may be altered ormodified and all such variations are considered within the scope andspirit of the present invention. In particular, every range of values(of the form, “from about a to about b,” or, equivalently, “fromapproximately a to b,” or, equivalently, “from approximately a-b”)disclosed herein is to be understood as referring to the power set (theset of all subsets) of the respective range of values, and set forthevery range encompassed within the broader range of values. Also, theterms in the claims have their plain, ordinary meaning unless otherwiseexplicitly and clearly defined by the patentee.

1. A method comprising: providing a treatment fluid comprising a basefluid and an orthoester-based surfactant; and introducing the treatmentfluid into a subterranean formation.
 2. The method of claim 1 whereinthe treatment fluid has a pH of about 8.5 or greater.
 3. The method ofclaim 1 wherein the base fluid is selected from the group consisting of:an aqueous fluid, an oleaginous fluid, and combinations thereof.
 4. Themethod of claim 1 wherein the orthoester-based surfactant is anemulsifying agent, a non-emulsifying agent, a foaming agent, a defoamingagent, a dispersant, a wetting agent, or a combination thereof.
 5. Themethod of claim 1 wherein at least a portion of the orthoester-basedsurfactant is of Formula I:

wherein R is hydrogen or an alkyl group having from 1 to 4 carbons, R¹is a hydrophobic group, R² is a hydrophilic group, R³ is CH₃, C₂H₅,C₃H₇, or C₄H₉, x is an integer from 1 to 2, y is an integer from 0 to 2,and z is an integer from 0 to 2, wherein the summation of x, y, and z isequal to
 3. 6. The treatment fluid of claim 5 wherein the hydrophobicgroup is derived from a hydrophobic alcohol that is capable ofundergoing a transesterification reaction with a low molecular weightorthoester of Formula II:

wherein R is hydrogen or an alkyl group having from 1 to 4 carbons, andR⁴ is an alkyl group having from 1 to 6 carbons.
 7. The method of claim5 wherein the hydrophobic alcohol is selected from the group consistingof a fatty alcohol, a fatty alcohol ethoxylate, an end-cappedhydrophobic poly(alkylene oxide), poly(tetrahydrofuran), andpolybutadiene hydroxyl terminated.
 8. The method of claim 5 wherein thehydrophobic group is an alkyl group with 8 or more carbons.
 9. Themethod of claim 5 wherein the hydrophilic group is derived from ahydrophilic alcohol selected from the group consisting of end-cappedpoly(ethylene oxide) or an alkanolamine.
 10. The method of claim 5wherein the hydrophobic group is derived from an end-capped hydrophobicpoly(alkylene oxide), and the hydrophilic group is synthesized fromend-capped poly(ethylene oxide).
 11. The method of claim 1 wherein theorthoester-based surfactant is synthesized from a diketene acetal or amultiketene acetal by the addition of a hydrophobic alcohol and ahydrophilic alcohol.
 12. The method of claim 1 wherein theorthoester-based surfactant contains an amine and is synthesized byreaction of an alkanolamine and a hydrophobic alcohol with a diketeneacetal or a multiketene acetal, or by transesterification of analkanolamine and a hydrophobic alcohol with a low molecular weightorthoester of Formula II:

wherein R is hydrogen or an alkyl group having from 1 to 4 carbons, andR⁴ is an alkyl group having from 1 to 6 carbons.
 13. The method of claim1 wherein the orthoester-based surfactant contains a tertiary amine. 14.The method of claim 1 wherein the orthoester-based surfactant is aquaternary ammonium compound.
 15. The method of claim 1 wherein theorthoester-based surfactant is present in the treatment fluid in anamount in the range of from about 0.01% to about 5% by weight of thetreatment fluid.
 16. The method of claim 1 further comprising the stepof: reducing the pH of the treatment fluid so as to facilitatedegradation of at least a portion of the orthoester-based surfactant.17. The method of claim 16 wherein the step of reducing the pH of thetreatment fluid comprises the step of: contacting the treatment fluidwith an acid.
 18. The method of claim 1 wherein the treatment fluid isintroduced into the subterranean formation at or above a pressuresufficient to create or enhance one or more fractures in thesubterranean formation.
 19. The method of claim 1 further comprising thestep of: drilling at least a portion of a well bore penetrating thesubterranean formation using the treatment fluid.
 20. A methodcomprising: providing an emulsified treatment fluid comprising anoleaginous phase, an aqueous phase having a pH of about 8.5 or greater,and an orthoester-based surfactant; and introducing the emulsifiedtreatment fluid into a subterranean formation.
 21. The method of claim20 wherein at least a portion of the orthoester-based surfactant is ofFormula I:

wherein R is hydrogen or an alkyl group having from 1 to 4 carbons, R¹is a hydrophobic group, R² is a hydrophilic group, R³ is CH₃, C₂H₅,C₃H₇, or C₄H₉, x is an integer from 1 to 2, y is an integer from 0 to 2,and z is an integer from 0 to 2, wherein the summation of x, y, and z isequal to
 3. 22. The method claim 20 wherein the emulsified treatmentfluid further comprises gravel particulates, and wherein the methodfurther comprises depositing at least a portion of the gravelparticulates within or adjacent to a portion of the subterraneanformation to form a gravel pack.
 23. The method of claim 20 furthercomprising the step of: reducing the pH of the aqueous phase so as tofacilitate degradation of at least a portion of the orthoester-basedsurfactant, thereby facilitating separation of the oleaginous phase andthe aqueous phase.
 24. The method of claim 20 further comprising thestep of: introducing the emulsified treatment fluid into thesubterranean formation at or above a pressure sufficient to create orenhance one or more fractures in the subterranean formation.
 25. Amethod of facilitating flow through a conduit comprising: providing anemulsified treatment fluid comprising an orthoester-based surfactant, anoleaginous phase, and an aqueous phase having a pH of about 8.5 orgreater; flowing the emulsified treatment fluid through the conduit; andreducing the pH of the aqueous phase so as to facilitate degradation ofat least a portion of the orthoester-based surfactant, therebyfacilitating separation of the oleaginous phase and the aqueous phase.26. The method of claim 25 wherein at least a portion of theorthoester-based surfactant is of Formula I:

wherein R is hydrogen or an alkyl group having from 1 to 4 carbons, R¹is a hydrophobic group, R² is a hydrophilic group, R³ is CH₃, C₂H₅,C₃H₇, or C₄H₉, x is an integer from 1 to 2, y is an integer from 0 to 2,and z is an integer from 0 to 2, wherein the summation of x, y, and z isequal to
 3. 27. The method of claim 25 wherein the oleaginous phasecomprises a heavy oil.
 28. A method of synthesizing an orthoester-basedsurfactant comprising: providing a diketene acetal or a multiketeneacetal, and reacting the diketene acetal or the multiketene acetal withone or more hydrophobic alcohols and one or more hydrophilic alcohols.29. The method of claim 28 wherein the step of reacting the diketeneacetal or the multiketene acetal is performed in the presence of acatalyst.